It is known to provide dynamic optical tagging systems based on a number of different technologies. Such tags have a wide variety of applications including, for example, identification and/or tracking of vehicles (road tolling), equipment (cargo container tracking), or people and in access control systems (for example in employee identification badges used to control access to specific areas). Such applications include both military and civilian use.
Known tagging systems have in common the basic concept of utilising a compact optically reflective tag which may be affixed, for example, to a vehicle, device, or person, etc. The tag may then be illuminated by means of a remote laser light source (preferably operating at a power and a wavelength which is not damaging to eyesight). Light reflected from tags in the area of illumination may then be detecting by means of a suitable optical detector and subsequently analysed so as to identify the location and possibly other information associated with those tags. Specific tag designs vary, and each has associated limitations.
Such tags may also incorporate the ability to modulate the reflected light so as to further identify the tag, or convey other information according to the complexity of programming of the tag. A number of ways is known to provide such modulation.
In particular U.S. Pat. No. 6,519,073 “Micromechanical Modulator and Methods for Fabricating the Same” (K. W. Goosen) and U.S. Pat. No. 5,500,761 “Micromechanical Modulator” (K. W. Goosen) disclose surface-normal micromechanical optical modulators. These modulators are arranged to reflect and modulate specific wavelengths only in a direction substantially normal to the surface of the device. Incident light arriving from a direction other than the normal to the surface of the device, will be reflected in a direction different from that of arrival. Furthermore, the wavelength of the light passed by such modulators varies as the angle of incidence deviates from the normal owing to the increase in optical path length within the device itself. For these reasons at least, such devices are in general not suitable for use as general purpose identification tags, particularly in situations in which an interrogating light source and detector may be arbitrarily located relative to the light-receiving surface of the tag.
Other known dynamic optical tags for such applications are based on a number of concepts based on optically switched retro-reflectors exploiting technologies such as liquid crystal and multiple quantum well modulators. Another approach utilises micro electromechanical corner cube retro-reflectors, the reflected signal being modulated by deforming the corner cubes which are fabricated using silicon microsystems. However, such techniques are unable to provide reliable communication links at data rates in excess of 50 kbps over a wide range of operating temperatures and the devices themselves costly to manufacture. Furthermore at ranges of, for example, 10 km between light source and reflector, calculations indicate that the diffuse return from background objects will be of a magnitude similar to that of any returns from such retro-reflectors. These returns act as a significant source of noise in the detection and interrogation process, thereby limiting both the effective data rate and the useful range of the devices.
A further disadvantage of known tags, especially those utilising liquid crystal components or multiple quantum well (MQW) components, is that their effective operating temperature range is undesirably limited. Consequently, in order to support reliable operation over a wide range to of temperatures, for example −40 C to +70 C, either explicit temperature stabilisation mechanisms are required, or the interrogator must be widely tunable in order to surmount the band-edge of the MQW material with ambient temperature.
Known tags based on combinations of refractive or diffractive mirrors or lenses also suffer from a restricted effective field of view which can make remote interrogation difficult.